MILK 19 



may undergo different varieties of fermentation when 

 acted on by microorganisms. Certain bacteria split up 

 lactose into lactic acid and certain by-products (carbon 

 dioxide, hydrogen, formic acid, butyric acid, etc. ) . These 

 organisms are the cause of the common "souring" and 

 curdling of milk. The bacteria of the coli-aerogenes 

 group ferment lactose and form acids and gases (lactic, 

 acetic, and succinic acids, carbon dioxide, carburetted 

 hydrogen, oxygen, and nitrogen) . 



Under normal conditions, lactose is found only in 

 the milk. If milk is retained in the udder from any 

 cause, as incomplete milking, omission of milking, udder 

 disease, etc., then lactose appears in the urine. When a 

 secreting udder is completely extirpated, glucose is tem- 

 porarily present in excess in the blood and appears in 

 the urine, while lactose appears in the urine if the udder 

 tissue is not completely removed. These latter facts are 

 taken to indicate that milk sugar is formed in the udder 

 from the glucose carried to it by the 'blood. 



Salts. The salts of milk, which are in solution, are 

 very largely inorganic. Calcium, potassium, and sodium, 

 together with small quantities of magnesia and oxide 

 of iron, are present in combination with phosphoric acid, 

 sulphuric acid, chlorine, and carbonic acid. A small por- 

 tion of the basic substances is in combination with citric 

 acid and probably with other organic acids. 



Water. The water of milk is derived from the blood. 

 The milk constituents, except the water, are referred 

 to as the milk solids, total solids, or dry matter. The 

 casein, albumin, globulin, lactose, and salts are desig- 

 nated as solids not fat. 



Variations in Composition. While normal milk always 

 contains the same chemical constituents, the proportions 



